A problem often encountered in the polymer film art relates to the difficulty of providing strong adhesion between substrates and functional coatings applied to them. This is particularly so in the case of polyester-based substrates. To deal with the problem, a primer layer or coating is generally applied to the polyester substrate to improve adhesion between the substrate and an overcoat applied to the substrate. For example, U.S. Pat. No. 4,098,952 (Kelly et al.) describes a primer layer useful for some polyester substrates comprising a copolymer comprising (i) 3 to 25 mole percent of glycidyl methacrylate or glycidyl acrylate, (ii) 35 to 95 mole percent of one or more other copolymerizable monomer selected from the esters of acrylic and methacrylic acids; and (iii) 1 to 60 mole percent of acrylonitrile. Other components can be added to the priming composition. For example, a crosslinking agent, such as alkyl melamines, may be used to crosslink with functional groups, such as hydroxyl and epoxy groups present in the composition. In addition, a catalyst can be used to accelerate the internal crosslinking action of the crosslinking agent and also to accelerate its crosslinking action with the crosslinkable functional groups in the priming copolymer. The priming copolymer can be applied to at least one surface of a support film, which can be any suitable self-supporting synthetic linear polyester film.
U.S. Pat. No. 5,391,429 (Otani et al.) describes a coating layer, useful as an adhesion priming layer for some polyester substrates, comprising at least 50% by weight of a water-soluble or water-dispersible polyester resin having a glass transition temperature of at least 20° C. One exemplary water-soluble or water-dispersible polyester resin is a copolyester having as its components terephthalic acid, sodium sulfoisophthalate, ethylene glycol, and diethylene glycol. The coating layer may optionally contain an additional water-soluble or water-dispersible resin, such as a water-soluble or water-dispersible acrylic resin. The acrylic resin may be in the form of a so-called core-shell polymer having different structures between an inner part and a surface part of each polymer particle.
A particularly difficult adhesion problem is encountered in the manufacture of optical films having a structured surface layer as describe in U.S. Pat. No. 5,908,874. The surface layer is formed from a composition that is polymerized from a radiation-polymerizable mixture of halogenated acrylate monomers. When this composition is coated onto a polyester film, poor adhesion of the coating composition to the polyester film is observed.
Certain sulfopolyesters are known to be useful as “primers” for polyester films. That is, when coated onto the surface of a polyester film, these sulfopolyesters enhance the adhesion of subsequent coatings. However, such known sulfopolyester primers have been found not to enhance sufficiently the adhesion to polyester films of the above-mentioned halogenated acrylate coatings.
Certain acrylate emulsion polymers are also known to be useful as primers for polyester films, and might be expected to provide considerable enhancement of adhesion of acrylate-based topcoats. However, it has been found that such acrylate primer coatings yield both insufficient enhancement of the adhesion to polyester films of the above-mentioned halogenated acrylate coatings, and considerable inconsistency in the measured level of adhesion. It is believed that this inconsistency, and the insufficient adhesion, are the result of the primer coating being dissolved or partially dissolved in the halogenated acrylate topcoat prior to curing of the topcoat.
Since known coatings fail to provide sufficient adhesion between all polyester substrates and all overcoats, especially acrylate-based overcoats, there still exists a need for novel chemical compositions for this purpose.